DC/DC Converter For A Subscriber Line Interface Circuit, System, Method &amp; Apparatus

ABSTRACT

A subscriber line interface circuit (SLIC) is herein described. In accordance with one aspect of the present proffered solution, the SLIC includes a first interface circuit operably coupled to a first subscriber loop and a second interface circuit operably coupled to a second subscriber loop. The SLIC further includes a power supply circuit that is coupled to the first and second interface circuits to provide first and second output voltages to the respective interface circuits. The power supply circuit includes a switching power converter that is operably supplied with an input voltage and is configured to provide an intermediate voltage signal at an intermediate circuit node. At least a first and a second output branch are connected to the intermediate circuit node. Each output branch includes an output capacitor that is coupled to the intermediate circuit node and that provides an output voltage signal that depends on the intermediate voltage signal. The switching power converter is configured to repeatedly set the signal level of the intermediate voltage signal to a high voltage level for a first time interval and to a low voltage level when the first time interval has elapsed.

SUMMARY

A DCDC converter is herein described. In accordance with one aspect ofthe present proffered solution, the DCDC Converter is applicable in asubscriber line interface circuit (SLIC) setting. The DCDC Converterincludes a first interface circuit operably coupled to a firstsubscriber loop and a second interface circuit operably coupled to asecond subscriber loop. There is further provided a power supply circuitthat is coupled to the first and second interface circuits to providefirst and second output voltages to the respective interface circuits.The power supply circuit includes a switching power converter that isoperably supplied with an input voltage and is configured to provide anintermediate voltage signal at an intermediate circuit node. At least afirst and a second output branch are connected to the intermediatecircuit node. Each output branch includes an output capacitor that iscoupled to the intermediate circuit node and that provides an outputvoltage signal that depends on the intermediate voltage signal. Theswitching power converter is configured to repeatedly set the signallevel of the intermediate voltage signal to a high voltage level for afirst time interval and to a low voltage level when the first timeinterval has elapsed.

Furthermore, a method for supplying power to a first and a secondsubscriber terminal is herein described, whereby the subscriberterminals are operably coupled to corresponding subscriber loops. Inaccordance with a further aspect of the present proffered solution, themethod comprises repeatedly charging a first and a second capacitor to ahigh voltage level during a first time interval, and preventingdischarging of the first and second capacitors to voltage levels lowerthan a given low voltage level when the first time interval has elapsed.The first and second capacitors provide power to the first and secondsubscriber terminals, respectively, via corresponding interface circuitscoupled to the respective subscriber terminals by the respectivesubscriber loops.

BRIEF DESCRIPTION OF THE DRAWINGS

The proffered solution can be better understood with reference to thefollowing drawings and descriptions. The components in the figures arenot necessarily to scale; emphasis is instead placed upon illustratingthe principles of the proffered solution. Moreover, in the figures, likereference numerals designate corresponding parts. In the drawings:

FIG. 1 is a block diagram illustrating an SLIC in a telephone exchangeconnected to a subscriber terminal located on the subscriber's premises;

FIG. 2 is a block diagram illustrating one exemplary implementation ofan SLIC in accordance with one example of the present profferedsolution;

FIG. 3 is a circuit diagram illustrating a conventional switching powerconverter circuit suitable for use in an SLIC;

FIG. 4 is a circuit diagram illustrating an improved switching powerconverter circuit for use in a two-channel SLIC in accordance with oneexample of the present proffered solution; and

FIG. 5 is a timing diagram illustrating the output voltages generated bythe switching power converter circuit of FIG. 3.

DESCRIPTION

Subscriber line interface circuits are typically used in telephoneexchanges (telephone switches) of telecommunications networks. Asubscriber line interface circuit (SLIC) provides a communicationinterface between a network operator's digital communication network andan analog subscriber line. The analog subscriber line connects to asubscriber station (e.g., a modem or a telephone, generally referred toas “subscriber equipment”) at a location remote from the exchange. SLICsare also used in cable modems and digital subscriber line (DSL) modems(or any other type of modem) where voice signals are received, e.g., viavoice over internet protocol (VoIP) and then converted into an analogvoice signal compatible with analog subscriber terminal equipment,particularly POTS terminal equipment (POTS=plain old telephone system).In this case the modem acts as a small “telephone exchange” remote fromthe central office. Today standard modems (e.g. cable modems), which arelocated in the subscriber's premises, include two interfaces forconnecting analog telephones.

The analog subscriber line and subscriber equipment form a subscriberloop. The SLIC usually needs to provide relatively high voltages andcurrents for control signaling with respect to the subscriber equipmenton the subscriber loop. For bi-directional communication, the SLICtransforms digital data received from the digital network into lowvoltage analog signals for transmission on the subscriber loop to thesubscriber equipment and vice versa. Generally, an SLIC usually usesdifferent power supply levels depending on the operational state of thesubscriber equipment. A first supply level is used when the subscriberequipment is “on hook” (standby mode) and a different, second supplylevel is used when the subscriber equipment is “off hook” (active mode).Two-channel or multi-channel SLICs provide the same functionality foreach channel. The channels operate independently but may share one powersupply.

SLICs previously employed power supply circuits with linear voltage (orcurrent) regulators to provide the different voltage (or current)levels, which is comparably inefficient in terms of power loss withinthe power supply circuit. More recent SLIC architectures use switchingpower converters (also known as switching-mode power supplies, or SMPS)to generate the appropriate supply voltage levels from a fixed DC inputvoltage. Although switching power converters are generally significantlymore efficient as they allow for significantly less power loss, there isstill a need for improvement to further reduce the power loss,particularly in the supply circuits of two-channel or multi-channelSLICs.

FIG. 1 illustrates the conventional use of a subscriber line interfacecircuit (SLIC). SLIC 21 is, for example, located in telephone exchange(telephone switch) 20, which may be located within a service provider'scentral office or a subscriber loop carrier (SLC). Generally, SLIC 21provides an interface between subscriber terminals 11 and 12 (e.g.,telephones), which are located on subscriber's premises 10, and theservice provider's (usually digital) telecommunications network, whichmay be part of public switched telephone network (PSTN) 30. In thepresent example, SLIC 21 includes two separate channels for interfacingwith the two separate subscriber terminals 11 and 12. The linesconnecting subscriber terminals 11 and 12 with SLIC 21 are usuallyreferred to as “subscriber loops” or “subscriber lines” and are labeledL11 and L12 in FIG. 1. Subscriber loops L11 and L12 are usuallyimplemented using twisted pair copper lines. Furthermore, SLIC 21includes an interface for connecting to a digital telecommunicationsnetwork, which is represented by PSTN 30 in FIG. 1.

Basically, subscriber terminals 11 and 12 may operate in two differentmodes, i.e., standby mode (“on-hook”) and active mode (“off-hook”). SLIC21 usually provides a relatively high DC voltage of, e.g., 42 to 48volts to a connected subscriber terminal if this subscriber terminal isin standby mode. However, the subscriber terminals do not consume asignificant supply current in standby mode. If the connected subscriberterminal is in active mode, SLIC 21 provides a constant current of about25 milliamperes, wherein the subscriber terminal has an “off-hook”resistance of approximately 300 ohms. SLIC 21 thus provides a relativelylow DC voltage of about 7.5 volts to a subscriber terminal in activemode. The mentioned current, voltage and resistance values have to beregarded as examples. The actual values are, however, usually defined byservice providers, regulating authorities or legislators and may bedifferent in different countries.

FIG. 2 illustrates the SLIC of FIG. 1 in more detail. In the presentexample, SLIC 21 includes the two interface circuits 211 and 212 forinterfacing to the corresponding subscriber loops L11 and L12,respectively. Interface circuits 211 and 212 are both configured toreceive signals from the connected subscriber terminals 11 and 12,respectively, to convert the signals in accordance with the usedtransmission standard and to transmit the converted signals totelecommunications network 30. Analogously, interface circuits 211 and212 are both configured to receive signals from telecommunicationsnetwork 30 to convert the signals appropriately to be transmitted to thedesired subscriber terminal via the corresponding subscriber loop. Insuch a manner, a bi-directional transmission is accomplished. To providethe voltage and current levels mentioned above, interface circuits 211and 212 are supplied using switching power converter 220. It should benoted that FIG. 2 is not a complete illustration of an SLIC. Componentsthat are known as such and that are not relevant for the presentdiscussion have been omitted for the sake of conciseness.

Switching power converter 220 may be a DC/DC converter that is suppliedwith input voltage V_(IN) and configured to generate first and secondoutput voltage signals V_(OUT1) and V_(OUT2). These output voltagesignals V_(OUT1) and V_(OUT2) are supplied to interface circuits 211 and212, respectively. The function and operation of switching powerconverter 220 will be described later with respect to FIGS. 4 and 5.

FIG. 3 illustrates the basic setup of a single-output switching powerconverter conventionally used in SLICs. When using single-outputswitching power converters in a two-channel (or multi-channel) SLIC,either one separate switching power converter has to be provided foreach channel or, alternatively, the same output voltage signal V_(OUT)has to be provided to both channels. Both options have undesiredconsequences. Providing two separate switching power converters is arather expensive solution, whereas using the same output voltage V_(OUT)for both channels (i.e., for both interface circuits 211 and 212)entails extensive power loss, particularly when the subscriber terminalconnected to the first channel is in active mode and the subscriberterminal connected to the second channel is in standby mode. In thiscase, the switching power converter would have to provide the highvoltage level (e.g., 42 volts) for the channel connected to thesubscriber terminal in standby mode and simultaneously provide therequired current (e.g., 25 mA) for the channel connected to thesubscriber terminal in active mode. Power loss higher than 2 watts maythus occur when only one of the two subscriber terminals is in activemode. In order to reduce power loss while simultaneously keeping costslow (by minimizing the number of expensive circuit components), theexemplary multi-output switching power converter 220 of FIG. 4 is usedin an SLIC, as discussed above with reference to FIG. 2. Beforediscussing FIG. 4, the basic switching converter of FIG. 3 is brieflyexplained.

Input voltage V_(IN) is applied across a series circuit of semiconductorswitch S₁ and inductor L₁. The series circuit is connected between aninput circuit node (at which input voltage V_(IN) is provided) and areference potential, e.g., ground potential. The common circuit node N₁of this series circuit (i.e., the middle tap between semiconductorswitch S₁ and inductor L₁) is connected to the cathode of diode D₁. Thecathode of diode D₁ is connected to circuit node N₂, which is coupled tothe reference potential, e.g., ground potential, via a first capacitorC₁. In essence, the output voltage could be tapped at circuit node N₂.However, an RC low-pass is used to reduce the output voltage ripple.That is, the output voltage is provided at circuit node N₃, which isconnected to circuit node N₂ via resistor R₁ and to the referencepotential via the second capacitor C₂. It can be seen that resistor R₁and the second capacitor C₂ form an RC low-pass. Output voltage V_(OUT)depends on input voltage V_(IN) and the switching operation ofsemiconductor switch S₁, which is usually controlled by a controllercircuit; this is known as such and thus omitted in the presentillustration of FIG. 3. Inductor L₁, semiconductor switch S₁, and diodeD₁, which has to be a fast recovery diode, are the most cost-intensivecircuit components. The first capacitor C₁ should be a ceramiccapacitor, whereas the second capacitor may be an electrolyticcapacitor.

The multi-output switching converter 220 of FIG. 4 includes a (e.g.,single-output) switching power converter circuit that provides oneintermediate voltage signal V_(C1) and two output branches fordistributing the voltage level of V_(C1) to the outputs of themulti-output switching converter 220. In the present example, themulti-output switching converter has two outputs providing first andsecond output voltage signals V_(OUT1) and V_(OUT2), respectively.However, further output branches could be provided dependent on theactual application. These output voltage signals V_(OUT1) and V_(OUT2)depend on the intermediate voltage V_(C1). The switching power convertercircuit, which provides the intermediate voltage signal V_(C1), includessemiconductor switch S₁, inductor L₁ and the first capacitor C₁, and isset up analogously to the previous example of FIG. 3. Accordingly, inputvoltage V_(IN) is applied across a series circuit of semiconductorswitch S₁ and inductor L₁. The series circuit is connected between aninput circuit node (at which input voltage V_(IN) is provided) and areference potential, e.g., ground potential GND. The common circuit nodeN₁ of this series circuit (i.e., the middle tap between semiconductorswitch S₁ and inductor L₁) is connected to the cathode of diode D₁. Thecathode of diode D₁ is connected to circuit node N₂, which is coupled tothe reference potential, e.g., ground potential, via a first capacitorC₁. The voltage signal across capacitor C₁ is the intermediate voltagesignal V_(C1) mentioned above. Therefore, circuit node N₂ could beregarded as the output of a single-output switching power converter thatprovides, as an output signal, the intermediate voltage signal V_(C1).The intermediate voltage signal V_(C1) depends on the input voltage andthe switching operation of semiconductor switch S₁, which is controlledby control circuit CTL.

Different from the previous example, circuit node N₂ is connected to twoseparate output branches. The first output branch includes diode D₂,resistor R₂ and a first output capacitor C₂, and the second outputbranch includes another diode D₃, another resistor R₃ and a secondoutput capacitor C₃. It should be noted that the diodes D2 and D3 can begenerally regarded as semiconductor switches which are configured toprevent a discharging of the output capacitors C₂ and C₃, respectively,via the circuit node N₂. The diodes D₂ or D₃ could be replaced by, e.g.,by appropriately driven semiconductor switches. The output circuit nodesof the output branches, at which output voltage signals V_(OUT1) andV_(OUT2) are provided, are denoted as circuit nodes N₃ and N₄,respectively. Accordingly, circuit node N₂ (at which the intermediatevoltage signal V_(C1) is provided) is connected to the first output nodeN₃ (at which the first output voltage V_(OUT1) is provided) via a seriescircuit of diode D₂ and resistor R₂. Additionally, circuit node N₂ isalso connected to the second output node N₄ (at which the second outputvoltage V_(OUT2) is provided) via a series circuit of diode D₃ andresistor R₃. Both output nodes N₃ and N₄ are connected to referencepotential via the output capacitors C₂ and C₃, respectively. Similar tothe previous example, resistor R₂ and capacitor C₂ form a first RClow-pass, while resistor R₃ and capacitor C₃ form a second RC low-pass.Diodes D₂ and D₃ prevent an undesired discharging of capacitor C₁ whenoutput voltages V_(OUT1) and V_(OUT2) are lower than voltage V_(C1) atcircuit node N₂. Output voltages V_(OUT1) and V_(OUT2) depend on inputvoltage V_(IN) and the switching operation of semiconductor switch S₁,which is controlled by control circuit CTL.

Control circuit CTL generates an appropriate drive signal forsemiconductor switch S₁, which may be of any type such as an MOStransistor or a bipolar junction transistor. For control purposes, thecontrol circuit may receive a feedback signal representing voltageV_(C1) or either of the output voltages V_(OUT1) or V_(VOUT2). Theoperation of the switching power converter is illustrated using thetiming diagram of FIG. 5. According to the example of FIG. 5, thecontrol circuit is configured to drive semiconductor switch S₁cyclically on and off such that the intermediate voltage signal V_(C1)is at a first level V_(H) (e.g., 42 volts) for a relatively short firsttime interval T_(H) (e.g., T_(H)=1 ms) and then at a second lower levelV_(L) (e.g. 7.5 volts) for a relatively long second time interval T_(L)(e.g., T_(L)=9 ms). In the present example, the first voltage levelV_(H) is generated regularly following a cycle time ofT_(C)=T_(H)+T_(L). However, the first voltage level is not necessarilyrepeated at a fixed frequency. One exemplary waveform of the generatedvoltage V_(C1) is illustrated in FIG. 5. It should be noted that theintermediate voltage V_(C1) and output voltages V_(OUT1) and V_(OUT2)may also be negative. It should also be noted that, when referring to a“high voltage level” or a “low voltage level”, it is the magnitude(absolute value) of the voltage level that is referred to. A voltagelevel of −40 volts is thus considered a high voltage level as comparedto a low voltage level of −7.5 volts.

For further discussion, it is assumed that output voltage V_(OUT1) issupplied to interface circuit 211 connected to subscriber terminal 11that is in active mode, whereas output voltage V_(OUT2) is supplied tointerface circuit 212 connected to subscriber terminal 12 that is instandby mode. Accordingly, approximately zero current is provided tosubscriber loop L12 (standby), whereas a desired load current (e.g., 25mA) is provided to subscriber loop L11 (active). During time intervalT_(H), capacitor C₁ and output capacitors C₂ and C₃ are charged toapproximately the same voltage V_(H) (when neglecting the voltage dropacross diodes D₂ and D₃). Therefore, output voltages V_(OUT1) andV_(OUT2) are at the desired high level V_(H) during time interval T_(H).During time interval T_(L), the intermediate voltage signal V_(C1) dropsto the low level V_(L). Nevertheless, output voltage V_(OUT2) remains ata relatively high level and drops only slightly during the subsequenttime interval T_(L) as the connected subscriber terminal 12 is instandby mode and sinks only a very low quiescent current.Simultaneously, subscriber terminal 12, which is in active mode, sinksthe desired current (e.g., 25 mA); capacitor C₂ thus dischargesrelatively quickly and, as a result, the corresponding output voltagedrops to (approximately) the low level V_(L). The correspondingwaveforms of output voltages V_(OUT1) and V_(OUT2) are also depicted inFIG. 5.

Employing the switching scheme illustrated in FIG. 5 allows asignificant reduction of power loss when only one of the two channels ofSLIC 21 (see FIG. 2) is active while the other is in standby mode. Theloss has been measured in a test setup and could be reduced from over 2watts (one output voltage V_(OUT) for both channels) to below 0.5 watts(multi-output switching converter of FIG. 4). Finally, it should benoted that the examples discussed above refer to a switching converterusing an inverting buck-boost converter topology. However, otherconverter topologies (e.g. flyback converter, inverting boost converter,etc.) may be applied dependent on the requirements of the intendedapplication.

Although various exemplary embodiments of the proffered solution havebeen disclosed, it will be apparent to those skilled in the art thatvarious changes and modifications can be made that will achieve some ofthe advantages of the proffered solution without departing from thespirit and scope of the proffered solution. It will be obvious to thosereasonably skilled in the art that other components performing the samefunctions may be suitably substituted. It should be mentioned thatfeatures explained with reference to a specific figure may be combinedwith features of other figures, even in those where not explicitly beenmentioned. Furthermore, the methods of the proffered solution may beachieved in either all software implementations using the appropriateprocessor instructions or in hybrid implementations that utilize acombination of hardware logic and software logic to achieve the sameresults. Such modifications to the inventive concept are intended to becovered by the appended claims.

1. An apparatus for a subscriber line interface, the apparatus includinga first interface operably coupled to a first subscriber loop, a secondinterface operably coupled to a second subscriber loop, and a powersupply coupled to the first and the second interface to provide a firstand a second output voltage to the first and second interface,respectively, the apparatus comprising: a switch unit operably suppliedwith an input voltage and configured to provide an intermediate voltagesignal; at least a first and a second output branch, each output branchprovides an output voltage signal that is dependent on the intermediatevoltage signal, wherein the switch unit is configured to set the signallevel of the intermediate voltage signal to a high voltage level for afirst time interval and to a low voltage level when the first timeinterval has elapsed.
 2. (canceled)
 3. The switch unit of claim 1,wherein each output branch includes an RC low-pass circuit.
 4. Theswitch unit of claim 1, wherein each output branch includes an LClow-pass circuit.
 5. The switch unit of claim 1, wherein each outputbranch comprises a diode and a resistor, which are coupled in series,the resistor and the output capacitor of each output branch forming anRC low-pass, and the diode connecting an intermediate node of the atleast first and second output branch and the RC low-pass.
 6. The switchunit of claim 1, wherein each output branch comprises a diode, acapacitor and an inductor, which are coupled in series, the inductor andthe capacitor of each output branch forming an LC low-pass circuit, andthe diode connecting to the LC low-pass circuit.
 7. The switch unit ofclaim 5, wherein, in each output branch, the diode is arranged such thatdischarging of the capacitor via the intermediate node is prevented bythe diode.
 8. The switch unit of claim 1, wherein the switch unit isconfigured to set a signal level of the intermediate voltage signalcyclically and with a cycle time to a high voltage level for a firsttime interval and to a low voltage level when the first time intervalhas elapsed.
 9. The switch unit of claim 8, wherein a ratio of the firsttime interval and the cycle time defines a duty cycle, and wherein theduty cycle is equal to or lower than 0.1 (10%).
 10. The switch unit ofclaim 8, wherein the first time interval is smaller than 10 ms and thecycle time is smaller than 100 ms.
 11. The switch unit of claim 1,wherein the switch unit includes an inductor, a semiconductor switch anda capacitor coupled to a downstream node providing a referencepotential.
 12. The switch unit of claim 11, wherein the inductor, thesemiconductor switch and the capacitor are arranged in accordance withan inverting buck-boost converter topology.
 13. A method for supplyingpower to a first and a second subscriber terminal operably coupled tocorresponding first and second subscriber loops, the method comprisingthe steps of: repeatedly charging a first and a second capacitor to ahigh voltage level during a first time interval, and preventingdischarging of the first and second capacitors to voltage levels lowerthan a given low voltage level when the first time interval has elapsed,wherein the first and second capacitors provide power to the first andsecond subscriber terminals, respectively, via corresponding interfacescoupled to the respective subscriber terminals by the respectivesubscriber loops.
 14. The method of claim 13, wherein charging the firstand second capacitors is initiated cyclically in accordance with a cycletime.
 15. The method of claim 14, wherein a ratio of the first timeinterval and the cycle time defines a duty cycle, and wherein the dutycycle is equal to or lower than 0.1 (10%).
 16. The method of claim 14,wherein the first time interval is smaller than 10 ms and the cycle timeis smaller than 100 ms.
 17. The method of claim 13, wherein the firstand second capacitors are each charged by a controllable voltage sourcevia corresponding first and second resistors and first and seconddiodes, respectively.
 18. The method of claim 17, wherein the diodeprevents a discharging of the first and second capacitors via thecontrollable voltage source.
 19. The method of claim 17, wherein thecontrollable voltage source is a switch unit.
 20. The method of claim17, wherein the first and second capacitors and the corresponding firstand second resistors form first and second RC low-pass filters.